You wait a lifetime for a Miliband, then two come along at once. Not my joke but one cracked by Miliband the younger, sympathising with the plight in which he and his brother have landed the Labour tribe, inflicting on them a dilemma that comes to a head this week as ballot papers drop through millions of letterboxes, demanding that Labour members and affiliated trade unionists finally make their choice.

Behind the quip is a recognition that if just one of them were in this contest, it would have been easy: current form suggests either one, David or Ed, would have left his rivals for dust. Instead, by fighting each other, they have turned this into a nailbiter, a contest that could well go down to the wire, perhaps settled – like Labour's 1981 deputy leadership contest – by a fraction of a single percentage point.

You'll note the assumption that Ed Balls is not likely to squeeze through the fraternal middle and take the prize. Yet he should not be dismissed. He has won a lot of admirers in this campaign, who now see a talent for both making the political weather – consistently steering stories on to the front pages – and landing steady blows on the government: Michael Gove has the scars to prove it. Over four months he has proved himself the most effective opposition politician in the Labour ranks. That he is trailing, according to the expert predictions, is a function of the dirt left on his hands after 16 years serving as Gordon Brown's most loyal lieutenant, as well as the long list of enemies he made doing his master's bidding. But this campaign has shown that he has serious brainpower and ability, establishing him as a Labour heavyweight to be reckoned with.

Still, the tealeaf readers are adamant: this is a family affair. By fighting each other to the finish the Milibands have highlighted imperfections in each other that might have gone unnoticed, illuminating nuances that would have remained invisible to all but the nerdiest observer. And they have left many a Labourite with an impossible desire: why can't we have the best of both?

In this realm of fantasy politics, the two would combine in a Labour leadership jobshare, bringing new meaning to the notion of "family-friendly working". Just as Bill and Hillary once told the American electorate it would get two Clintons for the price of one, so the Milibands could make a similar offer to Labour, each promising to fill the gaps left by the other. The Venus and Serena Williams of Labour would no longer fight each other on the singles court, but combine as a doubles pair.

So this two-headed Miliband would have all David's strengths, those qualities that saw him ensconced as frontrunner from the start. They begin with an aura of authority conceded even by his enemies. Some put it down to his height, even his well-tailored suits, but that is to omit five years of cabinet experience including three as foreign secretary. This gives him a heft that is an enormous asset, not least because – if the cuts bite as deeply as advertised – the next election might be winnable: in which case Labour needs to have a plausible, ready-for-office prime minister. Privately the Tories admit this is one reason why David Miliband is the would-be Labour leader they fear most.

Similar logic applies to the period before 2015. Labour was utterly rejected in May, with its lowest ever vote share bar the blowout of 1983. Before the electorate so much as begins to listen to Labour again, it will need to see the party led by someone it perceives as sensible, sober, credible. The older Miliband already cuts that figure.

David also best articulates what was a winning insight for New Labour in its earliest phase: that a way of reaching both its traditional base and beyond it was to speak on issues that some – especially those easily lampooned as Guardianistas – don't much like talking about, including crime, antisocial behaviour, welfare and immigration.

Last, and most surprising, any compound Miliband would be informed by the journey the older brother has taken in recent months. On Monday I saw him receive an ecstatic welcome from nearly a thousand community organisers, trained by his campaign. Modelled on the London Citizens movement, their programme – including the living wage – has been embraced by both brothers, but David has fallen for their grassroots methods, rightly hailing them as an entirely new way of doing politics.

But the double-headed Miliband would have Ed's strengths too. He would, by the limited nature of his cabinet experience, represent a break from the New Labour past. Where David is constantly asked to defend the record, saddled with the label of continuity candidate, Ed offers a newer face. He would still seem fresh-ish come 2015.

He would also be easier and more natural on TV, less angular, less apparently thin-skinned. Both brothers could have an Al Gore problem – being smart and right and even visionary, as Gore was in 2000, and yet still struggling to connect with an electorate looking for charm and likability – but Ed might have it in a smaller dose. He would, if the campaign is any guide, appeal to younger voters. He could also more rapidly win back those who fled to the Lib Dems over Iraq or New Labour's trampling on civil liberties, having made his own break with that part of the record. Above all he could speak to the millions of lower-income voters who, he has argued, abandoned Labour between 1997 and 2010 because their own living standards were badly squeezed, thanks in part to the party's embrace of a deregulated labour market.

If the Milibands were combined into a double act, each could compensate for the other's weaknesses. David suffers from his vote for the Iraq war: even though he says that had he known then what he knows now he would have voted against it, he has not made a clean break with that fatal and fateful decision. Ed has. David is dogged by questions about the treatment of British residents by foreign intelligence agencies, questions that the Tories would gleefully intensify with their inquiry into British complicity in torture; Ed has no such problem.

Ed could be dismissed as insufficiently substantive, one who has not given sufficient thought to, say, the question of how the deficit ballooned on Labour's watch – an answer to which will be essential if Labour is to be credible again – and who has preferred to tickle his party's most accessible erogenous zones. David fills that gap. If Ed has looked inward, focusing on the Labour selectorate, David has looked outward, acting as if the 2015 campaign against David Cameron had already begun.

In an ideal world, there would be a combined Miliband name on the ballot, blending the strengths of both. As it is, there are two imperfect, all too human individuals. Since only one can triumph, it is incumbent on the eventual winner to take on the arguments and qualities embodied by his defeated brother. The party has been offered an either/or choice. But the truth is, it needs both.